Good question. The stock '70 gears will be indicated by a code stamped into the axle tube. The stock power team chart below will give you a good idea. However over time many gearsets have been changed but most are probably stock.
Pulling the differental cover and counting teeth is the best way to see what is going on inside and what you have. Then you can confirm the code if it is stock or what has been changed. Any project should have the gear oil changed and inspecting the axles, gears, brakes, ect. is a easy project.
If you have not download the chassis overhaul manual and the service manual too.
http://67-72chevytrucks.com/vboard/s...d.php?t=558016